Photo: contributed
Some of the most important discoveries in science come from people who asked obvious questions, things so obvious that no one before bothered to ask.
Sir Issac Newton wondered why apples and other things fall. Olbers wondered why it was dark at night. Another deceptively simple question is why do the impact craters we see on the Moon, Earth, and elsewhere tend to be circular in shape?
A pitcher (in baseball) or a bowler (in cricket) can throw balls at speeds in excess of 150 miles per hour. This sounds fast, but compared to the speed of sound, which is about 1,200 miles per hour. This means, from a scientific point of view, the impact of this ball with the bat is a very slow process. The forces due to the impact recede through the ball at the speed of sound and the ball deforms smoothly. When the ball stops against the bat, it gets back in shape and shoots at high speed.
If no one catches the ball and touches soft ground, its very subsonic speed means that there will be a long time for it to push the ground out of the way, making a hole, and for the ball to warp and bounce. If the ball touches the ground almost vertically, it will make a round hole. On the other hand, if it touches the ground obliquely, it will make an elliptical mark or a gouge. However, if an object moves between 10 and 100 times the speed of sound, the story is very different.
We’re looking at a lab experiment. An aluminum sphere 25 mm in diameter is fired at a speed of 50 km / s in the direction of a thick aluminum slab. At this speed it takes one millionth of a second to travel 25 mm. When it touches the slab, there is no time for the slab material to move away because the forces have not had time to act.
There is no time for the impact forces to reach the back of the ball, so everything just piles up at the point of collision. The energy of the impact, about 29 million joules, is converted almost completely into heat, so that the metal ball most of the slab material is converted into an aluminum steam ball with a temperature of tens or even hundreds of thousands of degrees.
There is nothing in it that contains this steam and the internal pressure is huge, so it explodes outwards in all directions. It’s like someone just gently placed a very powerful bomb on the surface and then detonated it. Because the explosion is due to the ball of hot material, not its kinetic impact, the hole it makes is always round, even when the impact is at an angle. In the experiment, the slab ended up with a deep cup-shaped hole with a splattered metal fringe, resembling the image of something falling into the water.
A shock wave entered the slab and blew a piece off the other side.
The craters we see on Earth, the Moon and other objects in the Solar System are almost always circular due to the high impact velocity.
A ball of vaporized material is formed under enormous pressure, which then explodes in all directions. The shock wave pushes the ground down. When the shock wave has disappeared, the ground bounces off, forming a peak in the middle of the crater. Most craters have one of these central peaks.
Thinking about the violence of this experiment of aluminum balls, the energy release from the impact of an asteroid, for example, 10 kilometers in diameter, is difficult to grasp.
Some impacts on the Moon have splattered material over long distances. Tycho crater is a very obvious example. There are streaks of material that extend radially from the crater for hundreds of miles across the surface of the Moon.
Take out your telescope or binoculars and take a look at the Moon. Then remember that our world has been bombarded as well.
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• Mercury hides under the glow of dawn, with Venus to its right. Then, further to the right, are Mars, Jupiter, and then Saturn.
• The Moon will be new on June 28, and will reach its first quarter on June 6.
This article is written by or on behalf of a subcontracted columnist and does not necessarily reflect the views of Castanet.